To date, there are no experimentally clean processes identified that would allow verification or strong constraint of the calculations.
For example, the extraction of the photon PDF from isolated photon production in deep inelastic scattering (DIS) [1]}
or from inclusive \(pp\rightarrow \ell ^+\ell ^-+X\)  [2]}, [3]}, [4]} is limited due to large QCD background.
On the contrary, the elastic part of the photon PDF is verified via exclusive \(\gamma \gamma \rightarrow \ell ^+\ell ^-\)  process, measured in \(pp\)  collisions by ATLAS [5]}, [6]}, CMS [7]}, [8]} and recently by CMS+TOTEM [9]} collaborations.
